The use of high velocity air to atomize liquids, such as the production of a spray of fuel for combustion in gas turbines is well known, and the methods employed vary widely depending on the desired results in terms of fineness of atomization, the properties of the liquid fuel, the kind of penetration or dispersion of the spray cloud and the availability of air for the atomizing process.
For example, where compressed air can be supplied from an external source a device such as that disclosed in U.S. Pat. No. 3,474,970 can be employed, in which high velocity air is applied to one side of a conical fuel sheet produced by the discharge of a conventional spin-chamber or "simplex" nozzle flowing on the interior surface of a cone. The application of this principle, however, is limited to relatively low fuel flow rates and the nozzle operates as a conventional fuel pressure atomizer at high flows.
If the gas turbine is used in aircraft, the use of compressed air is generally not feasible and it is preferred to employ the air which is fed into the combustion chamber from the engine compressor to atomize the fuel. This method is disclosed in U.S. Pat. No. 3,283,502 which describes generally spreading the fuel into a thin film on a surface and atomizing the fuel sheet as it leaves the edge of this surface. U.S. Pat. No. 3,530,667 also shows the fuel being spread over a relatively large surface, with the atomizing air applied to both sides of the fuel sheet leaving the edge of the surface. Such fuel nozzles are conveniently described as the "prefilming" type. In both these cases, it is evident that the success of the atomization process can be affected by the behavior of the liquid film on the metal surface, since in general the size of drop produced is dependent on the thickness of the fuel film at the point of breakup. Variation of fuel film thickness can occur for various reasons and give rise to poor atomization performance in the following ways:
A. Viscous drag of the liquid on the surface will result in a decrease in velocity and therefore a thickening of the film. This effect obviously is aggravated by the use of a long flow path and higher fuel viscosities. The result is a general increase in drop size;
B. If the fuel is not spread evenly over the surface due to the method of introducing fuel in discrete jets then there will be locally thick regions which will result in large drops at these points;
c. If the air is in contact with the fuel film on the surface then surface waves may be produced which also cause local thickening of the film; and
d. If the air in contact with fuel has an irregular velocity distribution (such as that due to wakes downstream of swirl vanes) then the fuel film will be thickened locally from this cause.
It will be seen from the above that there are certain disadvantages in the methods disclosed which can operate to give fuel atomization which is unsatisfactory under many conditions.